Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of spinal stimulation has begun to expand to additional applications, such as angina pectoris and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory Parkinson's Disease, and DBS has also recently been applied in additional areas, such as essential tremor and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neurostimulation systems typically includes one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulation device implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. Thus, electrical pulses can be delivered from the neurostimulation device to the electrode(s) to activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. In particular, electrical energy conveyed between at least one cathodic electrode and at least one anodic electrode creates an electrical field, which when strong enough, depolarizes (or “stimulates”) the neurons beyond a threshold level, thereby evoking action potentials (APs) that propagate along the neural fibers. A typical stimulation parameter set may include the electrodes that are sourcing (anodes) or returning (cathodes) the modulating current at any given time, as well as the amplitude, duration, and rate of the stimulation pulses.
The neurostimulation system may further comprise a handheld patient programmer to remotely instruct the neurostimulation device to generate electrical stimulation pulses in accordance with selected stimulation parameters. The handheld programmer in the form of a remote control (RC) may, itself, be programmed by a clinician, for example, by using a clinician's programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
Of course, neurostimulation devices are active devices requiring energy for operation, and thus, the neurostimulation system may oftentimes includes an external charger to recharge a neurostimulation device, so that a surgical procedure to replace a power depleted neurostimulation device can be avoided. To wirelessly convey energy between the external charger and the implanted neurostimulation device, the charger typically includes an alternating current (AC) charging coil that supplies energy to a similar charging coil located in or on the neurostimulation device. The energy received by the charging coil located on the neurostimulation device can then be used to directly power the electronic componentry contained within the neurostimulation device, or can be stored in a rechargeable battery within the neurostimulation device, which can then be used to power the electronic componentry on-demand.
Typically, the therapeutic effect for any given neurostimulation application may be optimized by adjusting the stimulation parameters. Although the threshold for evoking action potentials may be a good indication of whether a desired therapeutic result is achieved, it is usually not directly observable when programming the neurostimulation device. For this reason, the programmer of the neurostimulation system is often required to identify the efficacy threshold and the side-effect threshold based on the patient's perception. For instance, the programmer of the neurostimulation system may identify the efficacy threshold by asking the patient whether the pain is relieved or perceived paresthesia, and record the set of stimulation parameters of that stimulation level. Similarly, the side-effect threshold is identified by adjusting the stimulation until the patient perceives any undesired side-effects such as slurred speech or involuntary muscle contraction, and records the set of stimulation parameters of that stimulation level. Then, the neurostimulation system is configured with a certain set of stimulation parameters to generate stimulation at an arbitrary level within the therapeutic window so that the stimulation is perceptible by the patient without causing any undesirable side effects.
There are a few issues that need to be considered when using this approach. Many neurostimulation therapies take time to develop the clinical benefit. For example, the patient may need to be on a certain level of stimulation for a few hours or even days before he or she can actually feel the pain relief or regain muscles mobility. Also, the side effect threshold is often not perfectly correlated with the therapeutic effect. Therefore, relying on the subjective clinical assessment (e.g., perception threshold) at the acute setting and configuring the stimulation parameters may result in an erroneous therapeutic window. Moreover, various changes, including postural changes, leads movement and tissue maturation, may occur in the patient during the course of therapy, and the stimulation parameters may need to be re-calibrated using the same unreliable subjective clinical assessment approach, thus the therapeutic window is often chosen to be very broad. That is, the gap between the efficacy threshold and the side-effect threshold is set as far as possible. In order to prevent under-stimulation and over-stimulation, a set of stimulation parameters are chosen to generate a stimulation pulse at the mid-level of the wide therapeutic window. The set of stimulation parameters for generating such stimulation pulse is more energy-intensive than necessary to achieve the therapy, which in turn causes decreased battery life, more frequent recharge cycles, and/or in the case where non-chargeable primary cell devices are used, more frequent surgeries for replacing the battery.
There, thus, remains a need to decrease the energy requirements for neurostimulation therapy.